Climate change and global warming is considered one of the most pressing and severe environmental problems of today. It is now generally accepted that the main cause for global warming is the release of the so-called greenhouse gases into the atmosphere. A major greenhouse gas is carbon dioxide (CO2), which is released predominantly from combustion of fossil fuels such as coal, petroleum and natural gas. Together, these fossil fuels supply about 80% of the energy needs of the world. Because fossil fuels are still relatively inexpensive and easy to use, and since no satisfactory alternatives are yet available to replace them on the enormous scale needed, fossil fuels are expected to remain our main source of energy in the long term.
One way to mitigate CO2 emissions and their influence on the global climate is to efficiently and economically capture CO2 from its source, such as emissions from fossil fuel-burning power plants and other industrial factories, naturally occurring CO2 accompanying natural gas, and the air. Once captured, CO2 can be sequestered in geological formations or under the sea, or can be used as a raw material to synthesize fuel and synthetic hydrocarbons.
Currently, separation and removal of CO2 from gas streams is achieved by techniques based on physical and chemical processes such as absorption by liquid solution systems, adsorption onto solid systems, cryogenic separation, and permeation through membranes.
Among various CO2 separation techniques, amine solution-based CO2 absorption/desorption systems are one of the most suitable for capturing CO2 from high volume gas streams. Commonly used solvents in such systems are aqueous solutions of alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and methydiethanolamine (MDEA). Certain sterically hindered amines, such as 2-amino-2-methyl-1-propanol (AMP), can also be used as absorbents because of their high CO2 loading capacities. Of these, MEA is most widely used because of its high CO2 absorption rate, which allows use of shorter absorption columns. However, MEA system presents major drawbacks, including the large amount of heat required to regenerate the solvent and operational problems caused by corrosion and chemical degradation. To prevent excessive corrosion, typically only 10 to 30 weight % MEA is used in an aqueous amine solution, with the rest being water. Because the entire solution, of which 70 to 90% is water, must be heated to regenerate the MEA system, a lot of energy is wasted during the regeneration process. Other alkanolamine systems also present disadvantages. For example, secondary and hindered amines (e.g., DEA, DIPA, AMP) provide more moderate CO2 absorption rates than MEA, and are also prone to corrosion and chemical degradation. MDEA is known to absorb CO2 only at a slow rate. Formulations formed by blending several alkanolamines are of interest because they can combine favorable characteristics of various compounds while suppressing in part their unfavorable characteristics. A number of blended alkanolamine solutions have been developed, and the most common blends are MDEA-based solution containing MEA or DEA. However, blended alkanolamine solutions do not eliminate the drawbacks of amine solution-based systems.
CO2 can also be captured by adsorption on solid sorbents. Solids are typically used as a physical adsorbent for separation of CO2. Such processes are based on the ability of porous solids to reversibly adsorb certain components in a mixture. The solids can have a large distribution of pore size, as in silica gel, alumina, and activated carbon, or a pore size controlled by the crystal structure, e.g., zeolites. At low temperatures like room temperature, zeolite-based adsorbents have high CO2 absorption capacities (e.g., 160 mg CO2/g for zeolite 13X and 135 mg CO2/g for zeolite 4A at 25° C. in pure CO2). However, the adsorption capacities of these adsorbents decline rapidly with increasing temperature. Further, because gases are only physically adsorbed on the adsorbents, actual separation of an individual gas from a mixture of gases is low.
To achieve a higher selectivity for CO2 adsorption, a compound providing chemical absorption can be applied on the solid adsorbent. For this purpose, an amine or polyamine can be deposited or grafted onto a solid support. Amines and polyamines chemically bound (grafted) on the surface of solids, such as silicas and alumina-silicas, however, show limited absorption capacity of less than 80 mg CO2/g and, in most cases, less than 50-60 mg CO2/g absorbent. For example, U.S. Pat. No. 5,087,597 to Leal et al. discloses a method for chemisorption of CO2 at room temperature using silica gel having a surface area between 120 and 240 m2/g, which is modified with a polyalkoxysilane containing one or more amino moieties in its structure. The material is disclosed to be capable of absorbing between 15 and 23 mg of dry CO2 per gram of absorbent. U.S. Pat. No. 6,547,854 to Gray et al. discloses a method for preparing amine-enriched sorbents by incorporating the amine onto the surface of oxidized solids. The reported maximum amount of CO2 absorbed on these solids is 7.7 mg/g absorbent using a gas mixture of 10% CO2 in He. As is evident from the data, the amount of CO2 that can be absorbed on the grafted amino group on various solid supports remains relatively low, because of their low amine coverage.
A more promising pathway involves impregnating a solid support with amines or polyamines. For example, a paper by S. Satyapal et al., J. Energy and Fuels 15:250 (2001) describe the development of polyethylenimine (PEI)/polyethylene glycol (PEG) on a high surface area polymethylmethacrylate polymeric support. This solid is currently used in space shuttles to remove CO2 from the cabin atmosphere and release it into the space. Its capacity is approximately 40 mg CO2/g absorbent at 50° C. and 0.02 atm. CO2. This material and its modifications are disclosed in U.S. Pat. Nos. 6,364,938; 5,876,488; 5,492,683; and 5,376,614 to Birbara et al. The preferred supports described in these patents are of polymeric nature, with acrylic ester resins such as AMBERLITE® being described as having particularly suitable characteristics. U.S. Pat. Nos. 5,376,614; 5,492,683; and 5,876,488 also disclose other possible supports, including alumina, zeolite and carbon molecular sieves. According to U.S. Pat. Nos. 5,492,683 and 5,376,614, however, the amount of amine present on the sorbent is limited, ranging from 1 wt. % to 25 wt. %.
U.S. Pat. No. 4,810,266 to Zinnen et al. discloses a method for creating CO2 sorbents by treating carbon molecular sieves with amine alcohols. This patent discloses that monoethanolamine (MEA)-based materials are not stable and release MEA during the regeneration step at higher temperatures. International Publication No. WO 2004/054708 discloses absorbents based on mesoporous silica supports. The active components for CO2 absorption are amines or mixture thereof chemically connected or physically adsorbed on the surface of the mesoporous silicas. Absorption on most of the absorbents described in this publication is below 70 mg CO2/g. The best results are obtained by using diethanolamine (DEA), which is physically adsorbed on the support (about 130 mg CO2/g). However, because of the volatility of DEA under the desorption conditions, the effectiveness of this absorbent generally decrease with increasing number of CO2 absorption-desorption cycle (about 16.8% after 5 cycles at a moderate regeneration temperature of only 60° C.). U.S. Pat. No. 6,908,497 to Sirwardane et al. discloses a method for preparing sorbents by treating a clay substrate having a low surface area of 0.72 to 26 mg2/g with an amine and/or ether.
Alcohols, polyethylene glycol and other oxygenated compounds have also been used for decades for acid gas removal, mainly CO2 and H2S. For example, SELEXOL® from Union Carbide (now Dow Chemicals) and SEPASOLV MPE® from BASF are used in commercial processes. Oxygenated compounds in combination with amines as mixed physical or chemical sorbents, in a process such as a glycol-amine process, have also been used for many years for acid gas removal (see Kohl, A. L. and Nielsen, R. B., GAS PURIFICATION 5th ed. (Gulf Publishing Co.)). U.S. Pat. No. 4,044,100 to McElroy demonstrates the use of mixtures of diisopropanolamine and dialkyl ethers of a polyethylene glycol for removing gases, including CO2 from gaseous streams. The use of ethylene glycol to improve the absorption and desorption of CO2 from amines has also been studied by J. Yeh et al., Energy and Fuels 15, pp. 274-78 (2001). While the literature mainly relates to the use of amines and oxygenated compounds in the liquid phase, the use of oxygenated compounds to improve characteristics of gas sorbents in the solid phase has also been explored. S. Satyapal et al., Energy and Fuels 15:250 (2001) mentions the use of polyethylene glycol in conjunction with polyethyleneimine on a polymeric support to remove CO2 from the closed atmosphere of a space shuttle. X. Xu et al., Microporous and Mesoporous Materials 62:29 (2003) shows that polyethylene glycol incorporated in a mesoporous MCM-41/polyethyleneimine sorbent improves the CO2 absorption and desorption characteristics of the tested material. Preparation and performance of a solid absorbent consisting of PEI deposited on a mesoporous MCM-41 is also disclosed (see X. Xu et al., Energy and Fuels 16:1463 (2002)). U.S. Pat. Nos. 5,376,614 and 5,492,683 to Birbara et al. use polyols to improve absorption and desorption qualities of the absorbents.
Another new material for trapping carbon dioxide are metal organic framework compounds. A preferred compound known as MOF-177 (J. Am. Chem. Soc., 2005, 127, 17998) has a room temperature carbon dioxide capacity of 140 weight percent at a relatively high pressure of 30 bar.
As these disclosures show, there is a need for an improved sorbent for capturing CO2, which is efficient, economical, readily available and regenerative, and which provides a high removal capacity at ambient as well as elevated temperatures. In addition, an efficient absorption system that solves the corrosion and evaporation problems of the existing technologies is needed.